Embodiments of the invention relate generally to x-ray imaging devices and, more particularly, to an x-ray tube having an improved cathode structure and improved control of electron beam emission.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The data acquisition system then reads the signals received in the detector, and the system then translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes typically include an anode structure for the purpose of distributing the heat generated at a focal spot. An x-ray tube cathode provides an electron beam from an emitter that is accelerated using a high voltage applied across a cathode-to-anode vacuum gap to produce x-rays upon impact with the anode. The area where the electron beam impacts the anode is often referred to as the focal spot. Typically, the cathode includes one or more filaments positioned within a cup for emitting electrons as a beam to create a high-power large focal spot or a high-resolution small focal spot, as examples. Imaging applications may be designed that include selecting either a small or a large focal spot having a particular shape, depending on the application.
It is desirable to deliver sub-microsecond mA modulation of the electron beam and/or gridding in some imaging applications. Some technologies are capable of increasing or decreasing electron beam amperage, but such technologies achieve mA modulation by changing the emitter temperature and thus the emitted beam current. Such mA modulation processes are often slow due to the thermal time constant of the emitter. That is, due to thermal mass of the filament, microsecond waveforms are difficult to obtain with this approach.
To achieve a fast mA response time, gridding technologies are often used to control electron beam operation electrostatically and modulate the mA, either via an intercepting or a non-intercepting grid. These gridding technologies may degrade the focal spot shape during mA modulation due to the presence of a gridding voltage. Such degradation is exacerbated when tube kV is modulated as well (as in, for instance, fast kV switching applications). Typically, if kV is increased or decreased, the mA will correspondingly increase or decrease as a consequence of respectively higher or lower electric fields at the emitter surface. These changes in kV and mA tend to impact the size and location properties of the focal spot during the changing operation.
In one example, a two-dimensional mesh grid is positioned between the cathode and the anode to modulate mA. Rungs of the mesh in the width direction tend to compress the beam more in its width, and corresponding rungs in the length direction tend to compress the beam more in its length. However, a two-dimensional grid tends to cause scatter in both length and width directions, and the amount of scatter is a function of an area of the rungs of the grid. Further, in many applications it is desirable to compress the beam width more than the beam length. Thus, in order to minimize scatter while enabling beam compression in the width dimension, a 1D mesh having rungs in the beam width direction may be implemented. Scatter may be reduced for a 1D grid by minimizing the individual width of the rungs in the 1D mesh and by increasing the length of each rung to ensure that any mount structure to which the rungs are attached are well clear of beam interference.
Because such grids are positioned in the electron beam, they are prone to heating due to deposition of electrons therein. The amount of heating may be reduced by reducing the voltage differential even to a slightly negative value therewith. Further, the amount of interference may be reduced by reducing the rung widths and increasing their lengths as stated above. Thus, not only may scatter be reduced by minimizing interference caused by the rungs, but the amount of heat deposited therein may correspondingly be reduced as well. Nevertheless, electrons are deposited therein during operation, and the electrons thus deposited cause the rungs to heat. Because the grid is positioned in a high vacuum, cooling of the rungs is limited to radiation and conduction modes of heat transfer. Radiant cooling tends to have an excessive time lag compared to the quick response of fast mA modulation. Conduction, likewise, is limited because the rate of conduction is a direct function of cross-sectional area of the rungs and inversely proportional to the length of the rungs. Thus, rungs in a 1D mesh are prone to excessive temperatures during operation, and the effect is aggravated as the rung width or thickness is minimized and as the rung length is increased as discussed above.
Heating and cooling of the rungs causes non-uniform thermal distortions to occur therein, which manifests itself in image quality artifacts and other image-related issues. As the rungs are narrowed in their width to reduce scatter and decrease deposited energy therein, they are, in comparison, made more flimsy and structurally weak. Accordingly, heating during mA modulation tends to non-uniformly distort the rungs, and the amount of distortion is driven by a number of factors that are exacerbated by thinning them. Distortion may manifest as, for example, bending and twisting of the rungs with respect to one another, the emitter, or the cup in which the emitter is mounted.
Therefore, it would be desirable to have an apparatus and method capable of microsecond mA modulation of an electron beam while maintaining image quality in an x-ray imaging device.